1. Field of the Invention
The present invention relates to amplifiers. More specifically, the present invention relates to high frequency RF amplifiers employing an envelope elimination and restoration technique (EER).
While the present invention is described herein with reference to a illustrative embodiment for a particular application, it is understood that the invention is not limited thereto. Those of ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope thereof.
2. Description of the Related Art
To improve the efficiency of linear radio frequency (RF) amplifiers certain techniques have been developed. Envelope elimination and restoration (EER) is one such technique. As described in "Single Sideband Transmission by Envelope Elimination and Restoration" by L. R. Kahn in the July 1952 Proceedings of the I.R.E., pp. 803-806 and in "Comparison of Linear Single-Sideband Transmitters with Envelope Elimination and Restoration Single-Sideband Transmitters" by L. R. Kahn, in the December 1956 Proceedings of the I.R.E., pp. 1706-1712, this scheme provides a method for amplifying a variable amplitude signal by separately amplifying its phase and envelope components. The input signal is amplitude limited so that a signal containing only phase modulation is amplified by the high efficiency RF amplifier. This permits the operation of a high efficiency RF amplifier, e.g. a Class C amplifier, at constant drive power. The envelope of the input signal is processed in a separate path to provide a modulated power supply for the RF amplifier. As the output of the RF amplifier varies in response to its supply, the envelope of the signal is restored as part of the properly amplified output signal.
In a typical system, the modulator includes an envelope detector and an envelope amplifier. If the modulator is of sufficiently high efficiency, the overall efficiency of the EER system may exceed that of the linear Class A or Class B amplifier that would otherwise be required to deal with the variable amplitude signal. The efficiency of the envelope amplifier is therefore a driver with respect to the efficiency of the overall system. Thus, variable duty cycle high switching speed converters have been used as envelope amplifiers. However, the switching speed of variable duty cycle high switching speed converters must be sufficiently higher than the signal bandwidth to permit removal of the switching frequency components by a suitable filter. For wide bandwidth signals, the switching frequency may be so high that the attendant switching losses significantly degrade the amplifier efficiency. This is a significant limitation on the use of variable duty cycle switching converters as envelope amplifiers.
There is therefore a need in the art for an envelope amplifier for use with an EER system which offers high frequency operation with significantly reduced switching losses.